Tilt meters or tilt sensors are used in the geophysical sciences and other applications to measure tilt or inclination of the ground or of structures. These measurements can be taken at the surface or, particularly in the case of geophysical applications, below the surface of the Earth in, for example, gas or oil wells. In these applications, the tilt meters are used to provide information about the general shape of a well or of sudden turns in the well. For example, oil wells can extend 10,000 or 30,000 feet below the surface of the Earth. At these depths, the wells can develop helical or corkscrew bores. These bores need to be monitored and tracked during the drilling process. In addition, highly sensitive tilt sensors are used in oil and gas wells to detect subtle changes in the Earth's structure, to measure subsidence, shifting or the quality or effects of nearby fracturing operations.
In general, tilt sensors are arranged to provide an indication of when the sensor changes orientation with respect to a predetermined reference, for example tilting with respect to horizontal, vertical or with respect to the direction of the Earth's gravitational pull. These sensors can provide a simple indication of tilt or can be calibrated to provide a measurement of the degree of tilt. Conventional tilt sensors are electronic devices that utilize a pendulum type sensor or a magnetic resistance element. Other types of tilt sensors include potentiometer type, servo type, bubble type, capacitance type and mercury type.
Other types of tilt sensors have combined optical elements with electronic elements. For example, U.S. Pat. No. 4,726,239 is directed to a soil analyzer and penetrator that includes a cone connected to the lower end of a hollow tube for measuring various ground parameters. A cone angle measuring device, which is an optical tilt meter, includes a light source, a photo sensor and a concave lens positioned between the light source and photo sensor. A steel ball rides on the concave surface of the lens. Therefore, the light from the source is limited in reaching the sensor depending upon the position of the ball. As the sensor is tilted, the ball rolls on the concave surface of the lens allowing more light to pass. Therefore, the output from the photo sensor is proportional to the angle of tilt of the sensor and thus of the cone.
Similarly, U.S. Pat. No. 5,134,283 is directed to an optical detection apparatus whereby the tilting state, such as rolling or pitching state, of an object, may be detected by a simplified optical system. The optical detection apparatus includes a light emitting element provided at distal end of a moveable member for radiating light downwards. A condenser lens is mounted below the moveable member, and a photo sensor device, for example a photodiode device, is mounted below the condenser lens. A signal processing circuit determines the position on the photo sensor device of a light spot from the condenser lens.
One system utilizes optical fibers in combination with conventional electronic optical detectors. U.S. Pat. No. 4,812,654 is directed to a two-axis quartz fiber passive tilt meter utilizing a quartz fiber suspended for emitting radiation from a distal end thereof and a pendulous mass suspended from the quartz fiber to improve the pendulous response. The infrared radiation transmitted and emitted by the quartz fiber is directed by a lens system to strike an axially displaced position on a detector producing DC signals representative of the intensity of light falling on the respective detector quadrants.
The combined systems still utilize electronic components that limits how small or compact the sensor can be. In addition, electronic sensors can be influenced by electromagnetic effects, temperature and signal attenuation, especially in deep well applications.
Therefore, a need exists for a tilt meter that is compact and suited for subsurface measurements. The tilt meter would obviate the need for electronic components located distally at the point of measurement, thereby eliminating electromagnetic interference.